Su Zemin, Maroncelli Mark
Department of Chemistry, The Pennsylvania State University, 104 Chemistry Building, University Park, Pennsylvania 16802, USA.
J Chem Phys. 2006 Apr 28;124(16):164506. doi: 10.1063/1.2189245.
Computer simulations are used to study solvation free energies and solubilities in supercritical solvents. Solvation free energies are calculated using the particle insertion method. The equilibrium solvent configurations required for these calculations are based on molecular dynamics simulations employing model solvent potentials previously tuned to reproduce liquid-vapor coexistence properties of the fluids Xe, C(2)H(6), CO(2), and CHF(3). Solutes are represented by all-atom potentials based on ab initio calculations and the OPLS-AA parameter set. Without any tuning of the intermolecular potentials, such calculations are found to reproduce the solvation free energies of a variety of typical solid solutes with an average accuracy of +/-2 kJmol. Further calculations on simple model solutes are also used to explore general aspects of solvation free energies in supercritical solvents. Comparisons of solutes in Lennard-Jones and hard-sphere representations of Xe show that solvation free energies and thus solubilities are not significantly influenced by solvent density fluctuations near the critical point. The solvation enthalpy and entropy do couple to these fluctuations and diverge similarly to solute partial molar volumes. Solvation free energies are also found to be little affected by the local density augmentation characteristic of the compressible regime. In contrast to solute-solvent interaction energies, which often provide a direct measure of local solvent densities, solvation free energies are remarkably insensitive to the presence of local density augmentation.
计算机模拟用于研究超临界溶剂中的溶剂化自由能和溶解度。使用粒子插入法计算溶剂化自由能。这些计算所需的平衡溶剂构型基于分子动力学模拟，该模拟采用先前经过调整以再现流体Xe、C₂H₆、CO₂和CHF₃的液-气共存特性的模型溶剂势。溶质由基于从头算计算和OPLS-AA参数集的全原子势表示。在不对分子间势进行任何调整的情况下，发现此类计算能够以±2 kJ/mol的平均精度再现各种典型固体溶质的溶剂化自由能。对简单模型溶质的进一步计算也用于探索超临界溶剂中溶剂化自由能的一般方面。对Xe的Lennard-Jones和硬球表示中的溶质进行比较表明，溶剂化自由能以及溶解度不受临界点附近溶剂密度波动的显著影响。溶剂化焓和熵确实与这些波动相关联，并且与溶质偏摩尔体积类似地发散。还发现溶剂化自由能几乎不受可压缩区域特有的局部密度增加的影响。与通常直接测量局部溶剂密度的溶质-溶剂相互作用能相反，溶剂化自由能对局部密度增加的存在非常不敏感。
